ver the last 40 years, underwater stereo-image measurement systems have employed stereo-cameras and paired single cameras in a variety of configurations. Both still and movie cameras have been used, based on film, video tube and digital sensors. Whilst measurement configurations of all varieties have been used, the transect has been, and still is, the predominant sampling technique. These basic approaches to photogrammetric geometry and sampling techniques are still in use today; however, the image quality has advanced considerably and the use of fully digital video systems and digital still cameras is now commonplace. The wider use of precalibrated, self-contained stereo-image systems, plus the substantial improvements in image resolution and image fidelity, is enabling new possibilities for accurate and reliable measurement of 3-dimensional lengths, surfaces and volumes. These advances are leading to significant improvements in the effectiveness of the management of marine ecosystems for conservation and the estimation of biomass for aquaculture. This work reviews the status of underwater stereo-image measurement and illustrates applications of stereo-image measurement in marine biology and ecology.